[Mesa-dev] [PATCH 06/11] loader_dri3: Honour the request to preserve back buffer content

[Thomas Hellstrom]

On 08/15/2017 09:33 AM, Michel Dänzer wrote:
> On 11/08/17 11:14 PM, Thomas Hellstrom wrote:
>> EGL uses the force_copy parameter to loader_dri3_swap_buffers_msc() to indicate
>> that it wants to preserve back buffer contents across a buffer swap.
>>
>> While the loader then turns off server-side page-flipping there's nothing to
>> guarantee that a new backbuffer isn't chosen when EGL starts to render again,
>> and that buffer's content is of course undefined.
>>
>> So rework the functionality:
>> If the client supports local blits, allow server-side page flipping and when
>> a new back is grabbed, if needed, blit the old back's content to the new back.
>> If the client doesn't support local blits, disallow server-side page-flipping
>> to avoid a client deadlock and then, when grabbing a new back buffer, sleep
>> until the old back is idle, which may take a substantial time depending on
>> swap interval.
>>
>> Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>
> [...]
>
>> @@ -475,12 +476,21 @@ dri3_find_back(struct loader_dri3_drawable *draw)
>>      int b;
>>      xcb_generic_event_t *ev;
>>      xcb_present_generic_event_t *ge;
>> +   int num_to_consider = draw->num_back;
>>   
>>      /* Increase the likelyhood of reusing current buffer */
>>      dri3_flush_present_events(draw);
>>   
>> +   /* Check whether we need to reuse the current back buffer as new back.
>> +    * In that case, wait until it's not busy anymore.
>> +    */
>> +   if (!draw->have_image_blit && draw->cur_blit_source != -1) {
>> +      num_to_consider = 1;
>> +      draw->cur_blit_source = -1;
>> +   }
> Is it possible that dri3_find_back gets called with
> draw->cur_blit_source != -1, entering this block (assuming
> !draw->have_image_blit)...
>
>
>>      for (;;) {
>> -      for (b = 0; b < draw->num_back; b++) {
>> +      for (b = 0; b < num_to_consider; b++) {
>>            int id = LOADER_DRI3_BACK_ID((b + draw->cur_back) % draw->num_back);
>>            struct loader_dri3_buffer *buffer = draw->buffers[id];
> ... Then later gets called again with draw->cur_blit_source == -1,
> choosing a different back buffer here, because the previous one is still
> busy?
>
>
I don't think that's possible. If dri3_find_back returns a back buffer, 
it should be idle, in the sense that it's not currently in the swap 
chain or being scanned out from. It may still have a non-signaled fence 
though. If a later call to dri_find_back() decides it's busy then 
someone must have called swapBuffer in between and it's time to change 
back anyway...

/Thomas